Methods to characterize sag in fluids

ABSTRACT

Systems and methods for direct and indirect measurement of the density of a fluid which exhibits sag characteristics is disclosed. The sag measurement system includes a test container for holding a fluid mixture to be analyzed and a suction port on the test container. A pump is coupled to the suction port for circulating the fluid mixture from the test container through a circulation loop. A measurement device is coupled to the circulation loop and a return port directs the fluid mixture from the circulation loop back to the test container at substantially the same vertical location as the suction port. The fluid mixture flowing through the circulation loop passes through the measurement device before returning to the test container through the return port. The measurement device is operable to monitor the particle distribution of the fluid mixture as it changes due to gravity.

BACKGROUND

Oilfield operations often entail the use of numerous fluid materialssuch as drilling fluids and fracturing fluids. A drilling fluid or “mud”is a specially designed fluid that is circulated in a wellbore orborehole as the wellbore is being drilled in a subterranean formation tofacilitate the drilling operation. The various functions of a drillingfluid include removing drill cuttings from the wellbore, cooling andlubricating the drill bit, aiding in support of the drill pipe and drillbit, and providing a hydrostatic head to maintain the integrity of thewellbore walls and prevent well blowouts. Specific drilling fluidsystems are selected to optimize a drilling operation in accordance withthe characteristics of a particular geological formation.

A drilling fluid typically comprises water and/or oil or synthetic oilor other synthetic material or synthetic fluid as a base fluid, withsolids in suspension. A non-aqueous based drilling fluid typicallycontains oil or synthetic fluid as a continuous phase and may alsocontain water dispersed in the continuous phase by emulsification sothat there is no distinct layer of water in the fluid. Such dispersedwater in oil is generally referred to as an invert emulsion orwater-in-oil emulsion.

A number of additives may be included in such drilling fluids and invertemulsions to enhance certain properties of the fluid. Such additives mayinclude, for example, emulsifiers, weighting agents, fluid-lossadditives or fluid-loss control agents, viscosifiers or viscositycontrol agents, and alkali.

The density of the drilling mud is closely maintained in order tocontrol the hydrostatic pressure that the mud exerts at the bottom ofthe well. If the mud is too light, formation fluids, which are at higherpressures than the hydrostatic pressure developed by the drilling mud,can enter the wellbore and flow uncontrolled to the surface, possiblycausing a blowout. If the mud is too heavy, then the hydrostaticpressure exerted at the bottom of the wellbore can reduce the rate atwhich the drill bit will drill the hole. Additionally, excessive fluidweights can fracture the formation causing serious wellbore failures. Insome cases, failure can cause drilling fluid to be lost to theformation, depleting the drilling fluid, leading to under pressurizationor well control problem. Thus, the control of the solids content of thedrilling fluid is very crucial to the overall efficiency and safeoperation of the rig.

In the most common applications, the density of the drilling mud isincreased by adding particulate weighting agents, such as barite andhematite. These particles are prone to settling within the drilling mudunder the influence of gravity. This settling is known in the industryas “sag” or “barite sag” and is a persistent and potentially seriousdrilling problem that occurs most prevalently in directional wellsdrilled with weighted drilling muds.

Sag can occur, for example, when circulation of the fluid is stopped fora period of time, e.g., when the drill string must be tripped from thewell, and is caused by the resulting settling or stratification of thefluid whereby “heavy spots” and “light spots” develop. Sag can alsoinvolve movement or shifting of these heavy and light fractions,particularly the “heavy spots,” where components such as barite havebecome concentrated. Sag may not occur throughout an entire well, butits occurrence in even a small section of the well can cause theproblems referred to below. Generally, higher temperatures exacerbatesag while higher pressures tend to retard sag.

Sag is not particularly problematic if the well is vertical or nearvertical. The magnitude of the problem may be smaller if the well, orthe section of the well in question, is nearly horizontal. However, ifthe well or a section thereof has a relatively high deviation angle(i.e., angle with respect to vertical), but falling well short of 90degrees, sag problems can become particularly severe. The advent andrecent strides in extended reach drilling, which have resulted inrelatively highly deviated wells, e.g., wells with deviation angles of20 degrees or more, has brought sag problems into focus in the drillingindustry.

Sag of the weighting agents in a fluid used in oil field operations cancause large density variations that often lead to significant wellborepressure management problems and potentially, wellbore failure.Additionally, fluid sag can lead to sticking of drill pipe, difficultyin re-initiating and/or maintaining proper circulation of the fluid,possible loss of circulation and disproportionate removal from the wellof lighter components of the fluid.

A number of solutions have been proposed for analyzing the sagproperties of a fluid. For instance, U.S. Pat. No. 6,584,833 to Jamisonet al. (hereinafter “'833 Patent”) discloses a method of determining thesettling rate of a fluid and is incorporated by reference herein in itsentirety. However, it is desirable to have a reliable method formeasuring and/or monitoring the sag of the weighting agents in thefield.

FIGURES

Some specific example embodiments of the disclosure may be understood byreferring, in part, to the following description and the accompanyingdrawings.

FIG. 1 is a Sag Measurement System in accordance with an exemplaryembodiment of the present invention.

FIG. 2 is a chart of the change in measured density over time for afluid sample obtained using a Sag Measurement System in accordance withan exemplary embodiment of the present invention.

FIG. 3 is a chart depicting the relationship between particle sizedistribution and material settlement for a sample analyzed in accordancewith an exemplary embodiment of the present invention.

While embodiments of this disclosure have been depicted and describedand are defined by reference to example embodiments of the disclosure,such references do not imply a limitation on the disclosure, and no suchlimitation is to be inferred. The subject matter disclosed is capable ofconsiderable modification, alteration, and equivalents in form andfunction, as will occur to those skilled in the pertinent art and havingthe benefit of this disclosure. The depicted and described embodimentsof this disclosure are examples only, and not exhaustive of the scope ofthe disclosure.

SUMMARY

The present invention is directed to systems and methods for monitoringfluids used in subterranean operations. More particularly, the presentinvention is directed to systems and methods for direct and indirectmeasurement of the density of a fluid which exhibits sagcharacteristics.

In one embodiment, the present invention is directed to a sagmeasurement system comprising: a test container for holding a fluidmixture to be analyzed; a suction port on the test container; a pumpcoupled to the suction port for circulating the fluid mixture from thetest container through a circulation loop; a measurement device coupledto the circulation loop; and a return port for directing the fluidmixture from the circulation loop back to the test container atsubstantially same vertical location as the suction port; whereingravity changes a particle distribution of the fluid mixture with time;wherein the measurement device is operable to monitor the particledistribution of the fluid mixture; and wherein the fluid mixture flowingthrough the circulation loop passes through the measurement devicebefore returning to the test container through the return port.

In another exemplary embodiment, the present invention is directed to amethod of analyzing sag performance comprising: placing a first fluidmixture to be analyzed in a test container; circulating the first fluidmixture through a first circulation loop including a density transducer;wherein the first fluid mixture enters and exits the first circulationloop through a first set of ports of the test container; and monitoringa change in density of the first fluid mixture as a function of time.

In another exemplary embodiment, the present invention is directed to amethod of characterizing sag performance of a fluid mixture comprising:determining a first density of the fluid mixture at a first point intime; inferring a first volume fraction of a solid component of thefluid mixture at the first point in time; determining a second densityof the fluid mixture at a second point in time; inferring a secondvolume fraction of the solid component of the fluid mixture at thesecond point in time; using the first volume fraction and the secondvolume fraction to determine the settling velocity of the solidcomponent in the fluid mixture.

In another exemplary embodiment, the present invention is directed to amethod of characterizing sag performance by determining a maximumparticle size of a solid component that may be suspended in a fluidportion of a fluid mixture, comprising: placing a fluid mixture in atest container having a lower portion and a higher portion; wherein ansuction port and a return port are placed at the lower portion of thetest container for directing the fluid mixture to a circulation loopincluding a density transducer; wherein the fluid comprises a testsection corresponding to the higher portion of the test container and amixed section corresponding to the lower portion of the test container;determining a first density of the fluid mixture in the mixed section ata first point in time using the density transducer; inferring a firstvolume fraction of the solid component of the fluid mixture in the mixedsection at the first point in time; determining a second density of thefluid mixture in the mixed section at a second point in time using thedensity transducer; inferring a second volume fraction of the solidcomponent of the fluid mixture in the mixed section at the second pointin time; determining a difference between the second volume fraction andthe first volume fraction; determining an increase in mass of the solidcomponent of the fluid mixture in the mixed section using the differencebetween the second volume fraction and the first volume fraction; usinga particle size distribution of the solid component and the increase inmass of the solid component in the fluid mixture in the mixed section todetermine the particle size range of the solid component that may besuspended in the fluid portion.

The features and advantages of the present disclosure will be readilyapparent to those skilled in the art upon a reading of the descriptionof exemplary embodiments, which follows.

DESCRIPTION

The present invention is directed to systems and methods for monitoringfluids used in subterranean operations. More particularly, the presentinvention is directed to systems and methods for direct and indirectmeasurement of the density of a fluid which exhibits sagcharacteristics.

The details of the present invention will now be discussed withreference to the figures. FIG. 1 depicts a Sag Measurement System(“SMS”) in accordance with an exemplary embodiment of the presentinvention, designated generally with reference numeral 100. The SMSincludes a test container 102 which holds the fluid to be analyzed. Aswould be apparent to those of ordinary skill in the art, with thebenefit of this disclosure, the container 102 may have any suitableshape and be positioned at angles other than vertical. In one exemplaryembodiment, the fluid may be a drilling fluid. In one embodiment, thetest container 102 may be heated and/or pressurized to simulatedifferent operating conditions. Moreover, in one embodiment, thecontainer 102 may have a means to shear or agitate the fluid beinganalyzed such that vertical mixing is minimized. For instance, in oneembodiment, the container 102 may contain a rotating concentric rodwhich would be operable to shear the fluid being analyzed at a known andcontrolled rate.

The test container 102 may have one or more sets of two ports 104, 106.One of the ports in each set is a suction port 104 and the other is areturn port 106. In some embodiments the function of the ports could bereversed periodically. The suction port 104 may be connected to a pump108 which circulates or reciprocates the drilling fluid being analyzedthrough the system. As would be appreciated by those of ordinary skillin the art, a number of different pumps may be used in the system. Forinstance, the pump may be a peristaltic pump, a gear pump, a centrifugalpump, a diaphragm pump or a piston pump. In all cases the circulation iscontrolled and the ports 104 and 106 are positioned such that thevertical mixing of fluid in container 102 is minimized. In the preferredembodiment, the ports 104 and 106 are tangentially aligned with insidediameter of 102. The pump 108 directs the drilling fluid through thecirculation loop 110. Specifically, the pump 108 directs the drillingfluid to a density transducer 112 and then back to the test container102 through the return port 106. Although FIG. 1 depicts a densitytransducer placed in the circulation loop 110, in another embodiment,other measurement devices may be used. For instance, the measurementdevice used may be one or a combination of a density transducer, arheometer or a viscometer. In some embodiments, multiple instrumentedloops may be used.

When using the SMS 100, the drilling fluid in the lower portion 114 ofthe test container 102 is continuously circulated through the system.This keeps the drilling fluid mixed so that no or minimal solids settleout of the fluid at the bottom of the test container 102 or the loop110. Additionally, the fluid in the upper part (unmixed) 116 of the testcontainer 102 is not affected by the flow and mixing in the lowersection 114. As the sag or settling process proceeds, the density in thelower section of the test container 102 increases. The rate at which thedensity in the lower section of the test container 102 increases is afunction of the sag resistance properties of the drilling fluid beingtested. Measurement loops above the bottom may have increasing ordecreasing density, depending on vertical location as well as the sagtendency of the test fluid at test conditions.

When using the SMS 100, a drilling fluid to be tested is placed in thetest container 102. The pump 108 then circulates the drilling fluidthrough the loop 110 and the density of the sample may be measured bythe density transducer 112. As the solid component of the drilling fluidsettles, the density of the drilling fluid being circulated through theSMS 100 increases. The change in the density of the drilling fluid maybe monitored as a function of time. In one embodiment, monitoring may beperformed by an automatic system such as, for example, a computerdevice.

In one embodiment, the data obtained from the SMS 100 may be used tocompare the sag performance of different drilling fluids. Specifically,a user may compare the change in density of different drilling fluidsusing the SMS 100 to determine which has a better sag performance. Inthis embodiment, once the density of a first fluid is monitored overtime in the SMS 100, the first fluid is removed from the test container102. The system may then be cleaned by passing a cleaning fluid throughit. As would be appreciated by those of ordinary skill in the art, withthe benefit of this disclosure, if the first fluid is oil based, anappropriate solvent may be used as the cleaning fluid. In contrast, ifthe first fluid is water based, then water may be used as the cleaningfluid. A second fluid may then be placed in the test container 102 andcirculated through the density transducer 112 of the SMS 100. The changein density of the second fluid over time may then be monitored and thedensity change of the two fluids may be compared to determine which hasa better sag performance. As would be appreciated by those of ordinaryskill in the art, with the benefit of this disclosure, the fluid whichhas a lower change in density over time has a better sag performance.Accordingly, a user may compare the magnitude of the differentialdensity of one drilling fluid to that of another drilling fluid as a wayto compare sag performance.

In one exemplary embodiment, the multiple sets of ports 104, 106 may beused to obtain similar measurements at different vertical locationswithin the container 102, allowing an analysis and monitoring of thechanges in the density of the drilling fluid at different locations inthe container 102. Depending on the user preferences, the measurementsat the different sets of ports 104, 106 may be obtained sequentially orsimultaneously. Although only three sets of ports 104, 106 are depictedin FIG. 1, as would be apparent to those of ordinary skill in the art,with the benefit of this disclosure, any numbers of sets of ports 104,106 may be used.

As would be appreciated by those of ordinary skill in the art, with thebenefit of this disclosure, there is a limit on the increase in thedensity of the drilling fluid at the bottom area of the test container102. This limit is due to the availability of solids that can settle inthe finite volume of the test container 102 as well as the maximumpacking density of the settled materials. Moreover, not all the solidparticles are the same size. As a result, all the solid particles do notsettle at the same rate and some may not settle at all.

As would be appreciated by those of ordinary skill in the art, with thebenefit of this disclosure, if the drilling fluids being compared havedifferent densities, the comparison becomes more difficult due to theavailability of the solids to settle. For instance, when comparing afirst drilling fluid with a low density to a second drilling fluid witha higher density, the settling rate of the solid components can actuallybe higher in the lower density drilling fluid. However, the totaldensity change of the samples over time may not accurately indicate thehigher settling rate of the barite in the first drilling fluid.

Accordingly, in another exemplary embodiment, fluids of variousdensities may be compared by calculating the average settling velocityof the solid component(s) of the drilling fluids. In one exemplaryembodiment, the solid component may be barite solids. The calculation ofthe settling velocity provides a method to compare results from fluidshaving different densities as well as providing a basis for comparisonto the Dynamic High Angle Settling Test which is disclosed in the '833Patent.

In this exemplary embodiment, the density of the drilling fluid is usedto infer a barite volume fraction. The change in the volume fraction ofthe barite is then used to calculate a settling velocity for the sample.The sum of volumes of the drilling fluid components is then used tocalculate a barite volume fraction. Specifically, the bulk fluid densityis obtained using the following equation:

ρ=φD _(barite)+(1−φ)(1−(% Oil/100))D _(brine)+(1−φ) (% Oil/100) D _(oil)  [Eq. 1]

where ρ is the bulk fluid density; φ is the volume fraction of settlingbarite and/or solids; % Oil is the percent volume of oil in the liquidphase. As would be appreciated by those of ordinary skill in the art,with the benefit of this disclosure, for an all-water fluid with no oil,% Oil is 0; for an all-oil fluids with no water or brine the % Oil is100; D_(barite) is the average density of the settling barite and/orother solids; D_(brine) is the density of the brine or water phase; andD_(oil) is the density of the oil phase. The above equation may berearranged as:

φ=(ρ−K)/(D _(barite) −K)   [Eq. 2]

where K=[(1−% Oil/100) D _(brine)]+[(% Oil/100) D _(oil)]

Accordingly, in one exemplary embodiment, the drilling fluid sample tobe analyzed may be put through the SMS 100 to determine the densitychange for a finite time. The data obtained may then be used tocalculate a settling velocity by making the following assumptions forthe case where the suction and the return ports are located at thebottom: (1) that the velocity of all the solids settling in the unmixedportion of the test container is a constant velocity; (2) that thechange in the barite volume fraction for the mixed portion is comprisedof the barite initially evenly distributed in the unmixed section of thetest container; (3) that all solids have the same specific gravity; and(4) that the settled fluid has not reached the maximum packing density.

Accordingly, the settling rate may be obtained using the followingequation:

[(φ_(t)−φ_(i))Vol_(mixed) ]/t=φ _(i) ×πR ² ΔX/t   [Eq. 3]

which can be rearranged as:

ΔX/t=V _(barite)=[((φ_(t)−φ_(i)) Vol_(mixed)]/ [φ_(i) πR ² t]  [Eq. 4]

where φ_(t) is the volume fraction of the barite and/or settleablesolids in the mixed portion at some elapsed time, t; φ_(i) is theinitial volume fraction of the barite and/or settleable solids;Vol_(mixed) is the volume of the mixed portion of the test system,including the volume of the lower portion 114 of the test container 102,the pump 108, the tubing loop 110 and the density transducer 112; R isthe radius of the test container; ΔX is the height of the unmixedportion of the test fluid in the container above the mixed zone that theaverage solid particle settles through to reach the mixed volume; andV_(barite) is the average settling rate of the barite and/or solids inthe sample.

As would be appreciated by those of ordinary skill in the art, with thebenefit of this disclosure the velocities obtained by this methodrepresent the average settling rate of the solids in the drilling fluidsample analyzed. Additionally, as discussed above and depicted in FIG.2, the density of the fluid sample being analyzed reaches a certainmaximum limit over time. As a result, the settling velocity calculatedin accordance with methods disclosed herein will also diminish with timeimplying that only a portion of the barite in the drilling fluid in theupper test section will settle.

In one exemplary embodiment, the PSD (Particle Size Distribution) curveof the barite (and/or other solid components used in a fluid to beanalyzed) may be used in conjunction with the system and methodsdisclosed herein to determine the maximum particle size which a testfluid can reliably suspend. As would be appreciated by those of ordinaryskill in the art, the particles having different sizes will not settleat the same rate. Therefore, assuming that all the density change in thelower portion 114 of the test container 102 is due to the larger andmore mobile particles, then the apparent particle size that does notsettle in the test fluid may be calculated. Once it is determined whatparticle size does not settle in the test fluid, that information may beused as a design method to determine the best grind size range forcommercial barite for sag performance.

The calculations used in determining the relationship between particlesize and settling may be carried out under the assumption that there waslittle or no particle attrition relative to initial PSD data of thefluid weighting agents and the settling particles were all of the samespecific gravity. However, if barite or other PSD data had been obtainedfor the fluid prior to testing, one could determine the particle size atwhich the fluid structure and particle to particle interactions wouldprovide sufficient settling inhibition for practical use.

As would be appreciated by those of ordinary skill in the art, thecomparison of the apparent non-settling particle diameter of one fluidmay be compared to that of another. That comparison may be used as atest measure to determine which particle size limit is best suited for agiven application. Specifically, the larger the particle size that maybe suspended by a fluid, the more sag resistant the fluid is. Moreover,as would be appreciated by those of ordinary skill in the art, thesystem and methods disclosed herein may be used to determine therelative volume fraction of the solid component that is mobile to thatwhich is not mobile. The results obtained using the system and methodsdisclosed herein may be used to determine the basic parameters requiredto develop a software model to determine the practical engineeringmodels for the sagging of solid particles.

In another exemplary embodiment of the present invention, in addition toobtaining density measurement, the SMS 100 may also be used to measurerheology of a fluid being tested. As would be appreciated by those ofordinary skill in the art, with the benefit of this disclosure, in thisembodiment, the viscosity of a fluid increases as the solids’ volumefraction increases due to barite sag.

Although the present invention is disclosed in the context of drillingfluids, as would be appreciated by those of ordinary skill in the art,with the benefit of this disclosure, the system and methods disclosedherein may be used in conjunction with any fluids where it would bedesirable to characterize and/or monitor fluid sag. For instance, thepresent invention may be used in conjunction with any particulate ladenfluid that exhibits particle sag or settling tendencies such ascementing fluids, spacer fluids, fracturing fluids and gravel packfluids. Moreover, although the present invention is disclosed as usingbarite as the solid component, as would be appreciated by those ofordinary skill in the art, with the benefit of this disclosure, thesolid component of the fluid being analyzed may be any solid suitablefor the intended application.

EXAMPLES I

An SMS in accordance with an exemplary embodiment of the presentinvention was used to analyze the sag performance of a drilling fluidwith barite as the weighing material. As depicted in FIG. 2, theinformation obtained from the density transducer of the SMS was thenused to plot the change in the density of the drilling fluid beingcirculated through the SMS as a function of time.

As depicted in FIG. 2, the density of the drilling fluid in the lowerarea of the test container increased from about 13.6 lb/gal to about15.5 lb/gal in about 30 hours. Moreover, the chart indicates that thedensity signature with time converges to a limit of about 15.5 lb/gal.

EXAMPLE II

The data set of Example I was used to determine the settling rate of thesolids in the sample. Specifically, the data obtained from the densitytransducer of the SMS over the first two hours of the analysis conductedin Example I was used to determine the settling velocity of the barite.The following values were used in making that calculation: % Oil=80;D_(brine)=1.231 g/cm³; R=3.81 cm; t=2 hours; Vol_(mixed)=476 cm³. Thefollowing table indicates the results obtained over the 2 hour period:

φ Settling Velocity_(barite) Time Density (lb/gal) (from Eq. 2) (mm/hr)16:36 13.598 0.257 18:36 14.038 0.275 3.9

EXAMPLE III

Using the system and methods disclosed herein, the size of the grains ina barite sample which would settle was determined. In this example, theupper portion of the test container which contained the unmixed portionof the fluid had a volume of about 2204 cm³. The lower portion of thetest container which contained the mixed portion of the fluid had avolume of about 476 cm³. Using Eq. 2, the initial and final density inthe mixed section and the volume fraction increase of barite was thencalculated as:

Test Section Volume=2204 cm³ Mixed Section Volume=476 cm³ Initial BariteVolume fraction=0.218 (assuming D_(barite)=4.23 g/cm³) Final BariteVolume fraction=0.287 Differential Volume fraction=0.068 D_(barite)=4.23g/cm³

Using the above values, when the density stopped increasing, theincrease in the mass of the barite in the test section, M_(increase),was calculated as:

M _(increase)=0.068 (476) 4.23=136.9 g

This mass increase represents 32.4 cm³ of barites. The initial baritevolume fraction was 0.218 of 471.7 cm³. As a result, 32.4 cm³ of the471.7 cm³ that was originally in the test section would have needed tosettle to increase the density. This indicates that about 6.9% of thebarite volume in the upper section would have settled. Assuming that thelarger particles settle faster than the smaller particles and that thereis no particle agglomeration, then all the settled volume would berepresented by particles in the larger size ranges of the PSD.Accordingly, as shown in the PSD curve of FIG. 3, particles with a sizeof about 50 microns and larger would have settled.

Therefore, the present invention is well-adapted to carry out theobjects and attain the ends and advantages mentioned as well as thosewhich are inherent therein. While the invention has been depicted anddescribed by reference to exemplary embodiments of the invention, such areference does not imply a limitation on the invention, and no suchlimitation is to be inferred. The invention is capable of considerablemodification, alteration, and equivalents in form and function, as willoccur to those ordinarily skilled in the pertinent arts and having thebenefit of this disclosure. The depicted and described embodiments ofthe invention are exemplary only, and are not exhaustive of the scope ofthe invention. Consequently, the invention is intended to be limitedonly by the spirit and scope of the appended claims, giving fullcognizance to equivalents in all respects. The terms in the claims havetheir plain, ordinary meaning unless otherwise explicitly and clearlydefined by the patentee.

1. A sag measurement system comprising: a test container for holding afluid mixture to be analyzed; a suction port on the test container; apump coupled to the suction port for circulating the fluid mixture fromthe test container through a circulation loop; a measurement devicecoupled to the circulation loop; and a return port for directing thefluid mixture from the circulation loop back to the test container atsubstantially the same vertical location as the suction port; whereingravity changes a particle distribution of the fluid mixture with time;wherein the measurement device is operable to monitor the particledistribution of the fluid mixture; and wherein the fluid mixture flowingthrough the circulation loop passes through the measurement devicebefore returning to the test container through the return port.
 2. Themeasurement system of claim 1, wherein the measurement device isselected from the group consisting of a density transducer, a rheometerand a viscometer.
 3. The measurement system of claim 1, wherein thefluid mixture is a drilling
 4. The measurement system of claim 1,wherein direction of flow of the fluid mixture through the circulationloop is reversible. 5-20. (canceled)